In a scanning electron microscope (SEM), a focused electron beam is scanned across the surface of a specimen. Electrons emitted from the region of the surface struck by the electron beam are detected, typically by a solid state diode or by a scintillator or phosphor that is optically coupled to a photomultiplier, and the current generated by the detected electrons determines the current of a beam of electrons in a cathode ray tube (CRT). As the focused electron beam is scanned over the surface, an image is built up on the CRT. SEM's are well known in the art and well described, for example, in numerous publications, starting with D. McMullan entitled “An improved scanning electron microscope for opaque specimens.” published in Proc. IEE vol. 100 Pt. 11, 245–259 (1953).
The faster electrons are called backscattered electrons (BSE) and the image formed is called a BSE image.